isdn/avm: fix build when PCMCIA is not enabled

Why wouldn't kconfig symbol ISDN_DRV_AVMB1_B1PCMCIA also depend on
PCMCIA?

Fix build for PCMCIA not enabled:

ERROR: "b1_free_card" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!
ERROR: "b1ctl_proc_fops" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!
ERROR: "b1_reset_ctr" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!
ERROR: "b1_load_firmware" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!
ERROR: "b1_send_message" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!
ERROR: "b1_release_appl" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!
ERROR: "b1_register_appl" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!
ERROR: "b1_getrevision" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!
ERROR: "b1_detect" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!
ERROR: "b1_interrupt" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!
ERROR: "b1_alloc_card" [drivers/isdn/hardware/avm/b1pcmcia.ko] undefined!

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Carsten Paeth <calle@calle.de>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: David S. Miller <davem@davemloft.net>